Gas phase reactors are commonly used for the polymerization of olefins such as ethylene and propylene as they allow relative high flexibility in polymer design and the use of various catalyst systems. A common gas phase reactor variant is the fluidized bed reactor. In polyolefin production, olefins are polymerized in the presence of a polymerization catalyst in an upwards moving gas stream. The fluidization gas is removed from the top of the reactor, cooled in a cooler, typically a heat exchanger, re-pressured and fed back into the bottom part of the reactor. The reactor typically contains a fluidized bed comprising the growing polymer particles containing the active catalyst located above a distribution plate separating the bottom and the middle zone of the reactor. The velocity of the fluidization gas is adjusted such that a quasi-stationary situation is maintained, i.e. the bed is maintained at fluidized conditions. In such a quasi-stationary situation, the gas and particle flows are highly dynamic. The required gas velocity mainly depends on the particle characteristics and is well predictable within a certain scale range. Care has to be taken that the gas stream does not discharge too much polymeric material from the reactor. This is usually accomplished by a so called disengagement zone. This part in the upper zone of the reactor is characterized by a diameter increase, reducing the gas velocity. Thereby the particles that are carried over from the bed with the fluidization gas for the most part settle back to the bed. Yet another fundamental problem with traditional fluidized bed reactors are the limitations as to the cooling capacity and entrainment due to the formation of huge bubbles. It should be mentioned that the presence of bubbles as such is desirable, since mixing is intensified thereby. However, bubble size should be much smaller than the diameter of the reactor. Increasing the bed level in conventional fluidized bed reactors for increasing the space-time yield leads to an increase of the bubble size and to an unwanted entrainment of material from the reactor. In conventional reactors there are no means for breaking up the bubbles.
Various modified gas phase reactor designs have been proposed. For example, WO-A-01/87989 has proposed a fluidized bed reactor without a distribution plate and an asymmetric supply of the reaction components to the reaction chamber.
Dual reactor assemblies comprising two reactors are also known. WO 97/04015 discloses two coupled vertical cylindrical reactors, the first reactor being operated under fast fluidization conditions. The first reactor having a frustoconical bottom zone and a hemispherical upper zone is coupled with the second reactor being a settled bed reactor. The operation under fast fluidization conditions is done in a reactor having a ratio of length/equivalent cross-sectional diameter of about 5 or more.
WO-A-01/79306 discloses a gas phase reactor assembly comprising a reactor including a distribution grid coupled with a cyclone separating solids and gaseous material. The separated solids are recycled back to the reactor.
WO-A-2009/080660 reports the use of a gas phase reactor assembly as described in WO-A-97/04015 comprising two interconnected reactors and a separation unit, the first reactor being a so called riser and the second reactor being a so called downcomer. The first reactor is operated under fast fluidization conditions.
However, the fluidized bed reactors and the dual reactor assemblies comprising a fluidized bed reactor described in the prior art still have several disadvantages.
A first problem concerns the plugging of the underside of the distribution plates due to entrainment of fines carried over with the circulation gas. This effect lowers operational stability and stability of the quality of the polymer. This problem partially can be overcome by lower fluidization gas velocity. However, a relatively low fluidization gas velocity limits the production rate and can lead to the formation of sheets, chunks and lumps in the production of polyolefins. This conflict of aims usually has been countered by the incorporation of a disengagement zone. However, disengagement zones again limit the production rate of a gas phase reactor of fixed size, as there is the need for additional top space above the top level of the fluidized bed during operation. In industrial dimensions, the volume of the disengagement zone often amounts to more than 40% of the total volume of the reactor and insofar requires the construction of unnecessary huge reactors.
A second problem concerns the bubbling. Conventional fluidized bed reactors typically operate in a bubbling regime. A part of the fluidization gas passes the bed in the emulsion phase where the gas and the solids are in contact with each other. The remaining part of the fluidization gas passes the bed in the form of bubbles. The velocity of the gas in the bubbles is higher than the velocity of the gas in the emulsion phase. Further, the mass and heat transfer between the emulsion phase and the bubbles is limited, especially for large bubbles having a high ratio of volume to surface area. Despite the fact that the bubbles positively contribute to powder mixing, formation of too large bubbles is undesired because the gas passing through the bed in the form of bubbles does not contribute to the heat removal from the bed in the same way as the gas in the emulsion phase and the volume occupied by the bubbles does not contribute to the polymerization reaction.
Yet a further problem concerns the entrainment of solids containing fines when removing the fluidization gas from the top of the reactor. Especially when operating the fluidized bed reactor with the bed level close to the roof of the reactor significant solid entrainment occurs. However, the presence of solids in the fluidization gas negatively affects the downstream units like compressors, heat exchangers, etc. Therefore, means are used to separate solids from the fluidization gas like for instance cyclones. Cyclones operate by taking advantage of the higher mass of the solids compared to the gas. Accordingly separation efficiency of the cyclone deteriorates with a decreasing mass of the solids. In other words increased amount of fines, i.e. small solid particles with a small mass, entrained from the reactor deteriorate cyclone efficiency as they are less efficiently removed by the cyclone than the larger fraction of the entrained solids.
Thus there is still the need for improved reactor design and operation. The present invention aims to overcome the disadvantages of the reactor designs known in the prior art and particularly aims to avoid the segregation of fines at a high production rate. The present invention further aims to increase the efficiency of separating solids from gas. The present invention further aims at avoiding low productivity zones in the reactor. Moreover, the present invention concerns the provision of a reactor, allowing high operational stability and at the same time production of polymer having highest quality.